1. Field of Invention
The invention relates to PWM (Pulse Width Modulation) and, in particular, to a Pulse Width Modulation method that is independent of supply voltages.
2. Related Art
In an ordinary DC brushless motor speed control system, the speed adjustment is controlled by the action time of the driving current on the coil. If the action time of the driving current is increased, the induced magnetic field continues longer. Therefore, the rotor in the motor is under the driving force for a longer time and becomes faster. On the other hand, if the action time of the coil current is decreased, the induced magnetic field exists a shorter time. Thus, the motor receives less interaction and rotates slower.
With reference to FIG. 1, part A shows that the driving voltage signal makes the current action time xc2xd of the cycle time; part B shows that the driving voltage signal makes the current action time xc2xc of the cycle time (xc2xe if the lower level provides the driving force); and part C shows that the driving voltage signal makes the current action time xc2xe of the cycle time (xc2xc if the lower level provides the driving force). Therefore, the motor speeds of those three driving signals have the relation C greater than A greater than B (or B greater than A greater than C if the lower level provides the driving force). In ordinary electronic circuit techniques, the PWM (Pulse Width Modulation) method is often used to control the action time of the driving voltage signals.
A typical PWM method is to send a series of triangular waves to a comparator. A reference voltage signal received by the comparator is then used as a standard to convert the triangular waves into square waves. FIG. 2 shows the relation between input and output signals of the comparator in the PWM fashion, where the two horizontal axes represent the time axes of the input and output signals at level 0. In the drawing, the curve v(t) is the input triangular wave signal and the curve vm(t) is the output reference voltage signal. The reference voltage signal is not necessary constant. The square wave vp(t) is the output signal from the comparator. It can be seen that when the triangular wave signal is greater than the reference voltage signal, the comparator outputs a certain positive voltage; while when the triangular wave signal is smaller than the reference voltage signal, the comparator outputs a certain negative voltage. The larger the reference voltage is, the smaller the output pulse width is; and the smaller the reference voltage is, the larger the output pulse width is. Therefore, from the magnitude of the reference voltage signal, one can control the pulse width of the output square wave. This method is the so-called PWM.
In the PWM method, that a stable triangular wave is provided as the base frequency input signal is the most important basic condition for performing PWM. Only when a stable triangular wave is provided can a required pulse width be made through a proper reference voltage. FIG. 3 shows a simple triangular wave generation circuit. The source and drain of a transistor 10 are connected to both ends of a capacitor 20. The source is connected to the ground. The drain is also connected to the input terminal of a level detector 30. Through a resistor 40, the drain is connected to a supply voltage VCC. The output terminal of the level detector 30 is connected to the gate of the transistor 10. Finally, the drain of the transistor 10 is further connected to a load VD.
When an external voltage VCC is supplied to the input terminal of the circuit, the capacitor 20 starts to be charged by the current from the resistor 40. Its terminal voltage rises and the transistor 10 now is open. If the reference voltage of the level detector 30 is smaller than the supplied voltage, and the output positive voltage is higher than the initial voltage of the transistor 10, then when the capacitor 20 is charged to the reference voltage the level comparator 30 outputs a positive voltage to the gate of the transistor. Therefore, the source and the gate of the transistor form a closed loop and the capacitor 20 discharges to its original state. FIG. 4 shows the curve of the load voltage versus time in this charge-discharge process. The curve AB represents the variation of the load voltage while the capacitor 20 is charged. In an extremely short time, the curve AB is almost a straight line and the slope is roughly the ratio between the supply voltage VCC and the resistance of the resistor 40. At point B, the load voltage reaches the reference voltage of the comparator 30. Thus, the transistor 10 forms a closed loop and the capacitor 20 starts to discharge, with the load returning to D. Repeating the charge-discharge process on the capacitor 20, a series of triangular waves can be form at the load terminal.
In the above-mentioned triangular wave generation process, the pulse width of the triangular waves is affected by the charging speed of the capacitor 20 (i.e. the voltage variation rate). The voltage variation rate of the capacitor is inversely proportional to the capacitance of the capacitor while proportional to the charging current. Since the charging current is governed by the supply voltage, when the supply voltage varies the charging current also changes, thus affecting the charging speed of the capacitor and the cycle time for generating triangular waves. In addition, the reference voltage of the comparator 30 is also changed by the supply voltage. These variation factors make the cycle time of triangular waves harder to be controlled. In motor speed controls, the control signal is limited by mechanical structures such as the motor rotor, stator coil and material selections. There is a preferred frequency range and thus no large variation is allowed. Therefore, how to use the PWM method to control the ratio of driving signals in different supply voltage ranges and the motor rotational speed is a complicated subject.
In light of the foregoing, we learn that applying the traditional PWM method to different supply voltage ranges will result in the problem of being hard to control the base frequency triangular waves cycle time. Therefore, the PWM control is extremely complex.
The present invention provides an operation speed control method for DC brushless motor. It adopts a supply voltage independent PWM method to adjust and control the motor speed. First, an electric current source linearly dependent upon voltage is used to charge a capacitor and the terminal voltage of the capacitor is coupled to a base frequency level comparator linearly dependent upon voltage. When the output voltage of the capacitor reaches the base frequency reference voltage, the signal output from the base frequency comparator makes the capacitor discharge. Between the charge and discharge of the capacitor, a series of triangular waves are output as the base frequency signal. Furthermore, under different supply voltages, all base frequency triangular waves thus generated have the same cycle time. The base frequency triangular wave is then sent to a speed control comparator. Through the PWM method, the output pulse width of the speed control comparator is adjusted by a speed control reference voltage according to the desired motor speed. The output pulse controls the rotational speed of the motor.
The disclosed supply voltage independent PWM device contains an current source that has a linear relation between its output current and supply voltage, a base frequency level comparator that has a linear relation between its reference voltage and supply voltage, a capacitor, an electronic switch, and a speed control comparator. The current source output a current to the capacitor. One terminal of the capacitor is connected to the ground while the other terminal is coupled to the base frequency level comparator and the speed control comparator. It is further coupled to the supply voltage through the current source. A switch connects between both terminals of the capacitor, forming a discharge loop. The switch is coupled to the output terminal of the base frequency level comparator. The conduction of the discharge loop is controlled by the output signal from the base frequency level comparator.